The present invention relates to a liquid crystal device for use in a liquid crystal optical switch, a liquid crystal optical shutter, etc., particularly to a liquid crystal device improved in display characteristics and electro-optical characteristics.
Heretofore, as a display apparatus for displaying various data or information, CRTs (cathode ray tubes) have been known and widely used for displaying motion pictures of television and video tape recorders or as monitor displays for personal computers. Based on the operation characteristic, however, the CRT is accompanied with difficulties such that the recognizability of a static image decreases due to flickering and scanning fringes caused by insufficient resolution, and the fluorescent member deteriorates due to burning. Further, it has been found that electromagnetic waves emitted from CRTs can adversely affect human bodies (e.g., health of VDT operators). Further, the CRT structurally has a large rearward space behind the display surface, so that the space economization in offices and at home may be obstructed thereby.
As a type of device solving such problems of the CRT, there has been known a liquid crystal panel (device), including a type using a twisted nematic (TN) liquid crystal as disclosed by M. Schadt and W. Helfrich, Appl. Phys. Lett., vol. 18, no. 4, pp. 127-128 (1971).
Liquid crystal devices using TN liquid crystal include a simple matrix-type liquid crystal device and an active matrix-type liquid crystal device wherein each pixel is provided with a TFT (thin film transistor).
The simple matrix-type liquid crystal device is advantageous from a viewpoint of production cost. This type of liquid crystal device is, however, accompanied with a problem that it is liable to cause crosstalk when driven in a multiplex manner using an electrode matrix of a high pixel density, and therefore the number of pixels is retracted. The problems of crosstalk and response speed can be solved by the active matrix-type liquid crystal device using TFTs, but, on the other hand, producing a larger area device of this type would be extremely difficult since inferior pixels are liable to occur. Further, even if such production is possible, the production cost would be increased enormously due to a lowering in production yield.
For providing improvements in light of the above-mentioned difficulties of the conventional types of TN liquid crystal devices, a liquid crystal device of the type which controls transmission of light in combination with a polarizing device by utilizing a refractive index anisotropy of ferroelectric (chiral smectic) liquid crystal (abbreviated as xe2x80x9cFLCxe2x80x9d) molecules has been proposed by Clark and Lagerwall (Japanese Laid-Open Patent Application (JP-A) 56-107216, U.S. Pat. No. 4,367,924). The ferroelectric liquid crystal (FLC) generally has chiral smectic C phase (SmC*) or H phase (SmH*) in a specific temperature range and, in the phase, shows a property of assuming either one of a first optically stable state and a second optically stable state in response to an electric field applied thereto and maintaining such a state in the absence of an electric field, namely bistability, and also have a very quick response speed because it causes inversion switching based on its spontaneous polarization. Thus, the FLC develops bistable states showing a memory characteristic and further has an excellent viewing angle characteristic. Accordingly, the FLC is considered to be suitable for constituting a high speed, high resolution and large area display device.
In the FLC panel (device), at an initial alignment stage, liquid crystal molecules placed in a first stable state and those placed in a second stable state are co-present in a domain. More specifically, in a device using a chiral smectic liquid crystal developing bistable states, an alignment control force for aligning liquid crystal molecules to be placed in a first stable state and that for aligning liquid crystal molecules to be placed in a second stable state have a substantially equal energy level. As a result, when the chiral smectic liquid crystal is disposed between a pair of substrates, each provided with an alignment film in a thickness sufficiently small to assume bistability, resultant oriented (aligned) liquid crystal molecules in a domain include a portion placed in a first stable state and a portion placed in a second stable state in combination at an initial alignment stage.
Further, similar to the FLC device, a liquid crystal device of the type wherein a refractive index anisotropy and a spontaneous polarization of liquid crystal molecules are utilized, there has been known a liquid crystal device using an anti-ferroelectric liquid crystal (abbreviated as xe2x80x9cAFLCxe2x80x9d). The AFLC generally has chiral smectic CA phase (SmCA*) in a specific temperature range and, in the phase, shows a property of assuming an average optically-stable state, wherein liquid crystal molecules are oriented in a direction of a normal to smectic (molecular layers under no electric field application and tilting its average molecular axis direction from the layer normal direction under application of an electric field). Further, the AFLC causes switching based on its spontaneous polarization in combination with the applied electric field, thus exhibiting a very quick response. Accordingly, the AFLC is expected to be used in a high speed liquid crystal panel.
As one of the conventional liquid crystal devices, a transmission-type liquid crystal panel (device) will be described with reference to FIG. 5.
FIG. 5 is a schematic sectional view showing a portion closer to a boundary between a display region and a peripheral region of an embodiment of a conventional liquid crystal panel.
Referring to FIG. 5, a liquid crystal display panel (liquid crystal device) includes a pair of oppositely disposed glass substrates 111 and 111 which are bonded to each other via a sealing agent 112 at the periphery thereof with a gap. The gap is held by spacer beads 115 and filled with a chiral smectic liquid crystal 113. On the surface of each of the glass substrates 111 (111), a laminated film 114 comprising, e.g., a transparent (ITO: indium tin oxide) electrode, an insulating film, an inorganic oxide insulating film and an aligning-treated alignment film for aligning the liquid crystal 113 sequentially disposed on the glass substrate is formed. In the peripheral region of the device, a frame-shaped masking (light-interrupting) member 116 (so called xe2x80x9cblack matrixxe2x80x9d) comprising, e.g., a metal is disposed so as to surround the display region.
The liquid crystal display panel may generally be driven by applying a voltage of at least a certain threshold value to the liquid crystal 113. In this case, if a cell gap between the glass substrates 111 (111) is not uniform over the entire picture area, an electric field applied to the liquid crystal 113 becomes nonuniform to cause image irregularity and irregularity in driving characteristics, thus resulting in inferior image qualities. Particularly, in the case of a liquid crystal display panel using the FLC or AFLC, it is necessary to provide a small cell gap of ca. 1-3 xcexcm. Accordingly, even when a degree of nonuniformity of cell gap is slight, the resultant nonuniform cell gap significantly affects image quality.
In order to keep such a small cell gap uniform, it has been proposed to use spacer beads (spherical spacer) 115 or stripe spacers formed through flexible printing, photolithography or dry film.
The latter stripe spacers employed in a liquid crystal display panel may generally be formed in a pattern such that each stripe line extends from a liquid crystal injection port with an identical width, a gradually increasing width or a gradually decreasing width. The stripe spacers may generally be formed with a material having a prescribed viscosity so that each stripe line has an identical width and a width as large as possible.
On the other hand, the former spacer beads 115, as shown in FIG. 5, may ordinarily be dispersed uniformly over the entire display panel. As a result, under the frame-shaped masking member 116 providing a protruding surface, the spacer beads 115 may be located, thus resulting in nonuniform cell gap distribution between the display region inside the masking member 116 and the peripheral region including the masking member 116 as shown in FIG. 5.
In order to solve the problem, the liquid crystal display panel using the spacer beads is required to modify its layer structure ox a pattern of metal electrodes so as not to cause a nonuniform cell gap, thus leading to many constraints on cell structure design.
Further, the latter stripe spacers function as a cell gap-regulating member when a pair of substrates are applied to each other under a certain pressure. In that case, a portion of the stripe spacers contacting the frame-shaped masking member 116 is not readily deformed compared with other portions. As a result, in the peripheral region where the masking member 116 is formed, e.g., the (upper) glass substrate 111 is liable to be extruded, thus failing to provide a uniform cell gap over the entire display panel.
Further, when an adhesive (contact) area per unit substrate area between the glass substrate 111 and the stripe spacers in the peripheral region including the masking member 116 is different from that in the display region including an electrode matrix, the resultant cell gap distribution is liable to be changed, thus leading to a nonuniform cell gap.
Next, another problem of conventional liquid crystal devices will be described.
The above-mentioned FLC includes one showing a characteristic such that a response speed or time (xcfx84) of a liquid crystal (FLC) provides a minimum (xcfx84Vmin) when a voltage (V) applied to the liquid crystal is increased (hereinbelow, referred to as xe2x80x9cxcfx84Vmin characteristicxe2x80x9d). The liquid crystal of this type (xcfx84Vmin mode) shows a negative dielectric anisotropy (xcex94"xgr" less than 0) or a positive biaxial dielectric anisotropy (xcex94"xgr" greater than 0) and provides a xcfx84Vmin characteristic based on a torque due to the dielectric anisotropy for stabilizing the FLC larger than that due to inversion switching of the FLC. The FLC showing the xcfx84Vmin characteristic allows the resultant liquid crystal device to provide a high brightness, a high contrast and a high speed responsiveness. Further, some of the AFLCs described above have the xcfx84Vmin characteristic.
FIG. 13 is a schematic sectional view showing a part of the production steps of a conventional liquid crystal device using the FLC or AFLC of xcfx84Vmin mode or the above-described another FLC or AFLC in combination with spacer beads 50.
Referring to FIG. 13, the spacer beads 50 are dispersed on one (a substrate 51b in this embodiment) of a pair of substrates 51a and 51b, each having thereon a plurality of films such as ITO electrode, organic or inorganic (oxide) films, and alignment films as mentioned above with reference to FIG. 5. In FIG. 13, reference numerals for the films are omitted for simplicity of explanation.
These spacer beads 50 can provide a relatively smaller cell gap (e.g., 1-3 xcexcm) if particle sizes thereof are made uniform. However the spacer beads 50 are generally dispersed uniformly on one of the substrates in many cases, so that a part of the spacer beads 50 may be located at pixel portions for display in the display region, thus being liable to cause alignment defects in the vicinity of such spacer beads to result in insufficient contrast of the resultant liquid crystal device.
In addition, the spacer beads 50 may be used in combination with adhesive beads 52 for improving an adhesive strength between the pair of substrates 51a and 51b as shown in FIG. 13.
The adhesive beads 52, however, similar to the spacer beads 50, are liable to cause alignment defects which lower contrast of the resultant liquid crystal device.
FIG. 14 is a schematic sectional view showing a part of the production steps of a conventional liquid crystal device employing stripe spacers 53.
Referring to FIG. 14, the stripe spacers 53 may generally be formed selectively at non-pixel portions using a photolithographic process. As a result, the stripe spacers 53 are not present at pixel portions for display, thus being liable to cause few alignment defects of a liquid crystal used.
The stripe spacers 53 may be provided with an adhesive function in addition to the cell gap-regulating function, thus remarkably decreasing the number of alignment defects compared with the case of using the adhesive particles 52.
However, when such advantageous stripe spacers 53 are used in a liquid crystal device shown in FIG. 15, it has been found that the display region includes a C1 alignment region and a C2 alignment region, and the C2 alignment region is liable to appear on a downstream side of a rubbing direction A, specifically a region ranging from one end (distant from a liquid crystal injection port) of each stripe spacer 53 to a position ten to several ten millimeters apart therefrom. This may be attributable to disorders in flow of the injected liquid crystal in the vicinity of the ends of the stripe spacers 53 and phase transition thereof. In the C1 alignment region, drive (switching) of the liquid crystal is not performed sufficiently and a lowering in contrast is caused.
A principal object of the present invention is to provide a liquid crystal device having solved the above-mentioned problems.
A first object of the present invention is to provide a liquid crystal device capable of suppressing an unevenness of a cell gap to provide uniform driving characteristics in the entire display area.
A second object of the present invention is to provide a liquid crystal device capable of substantially suppressing an occurrence of alignment defects to improve contrast.
According to the present invention, in view of the first object, there is provided a liquid crystal device comprising a pair of substantially parallel substrates disposed opposite to each other to leave a gap extending two-dimensionally therebetween including a rectangular wider gap region and a peripheral frame-shaped narrower gap region surrounding the wider gap region, a plurality of spacers disposed in the gap over the wider and narrower gap regions, and a liquid crystal disposed between the pair of substrates so as to form pixels in the wider gap region, wherein the spacers are disposed to have a larger volume per unit substrate area in the wider gap region and a smaller volume per unit substrate area in the narrower gap region.
In another aspect in view of the first object, according to the present invention, there is also provided a liquid crystal device comprising a pair of substantially parallel substrates disposed opposite to each other to leave a gap extending two-dimensionally therebetween including a rectangular wider gap region and a peripheral frame-shaped narrower gap region surrounding the wider gap region, a plurality of spacers disposed in the gap over the wider and narrower gap regions, and a liquid crystal disposed between the pair of substrates so as to form pixels in the wider gap region, wherein the spacers are disposed to have a spacing between the spacers and one of the substrates in the narrower gap region.
According to the present invention, in view of the second object, there is provided a liquid crystal device comprising a pair of substrates each provided with a plurality of stripe electrodes and an alignment film having a uniaxial alignment axis for aligning a liquid crystal, a plurality of stripe spacers disposed between the substrates in parallel with stripe electrodes provided to at least one of the substrates, and a liquid crystal layer comprising a ferroelectric or anti-ferroelectric liquid crystal disposed together with the stripe spacers between the substrates, wherein the plurality of stripe spacers are connected with a connecting spacer at their ends along one side so as to block each spacing between adjacent stripe spacers.